Magma (English)

5 - 3

How is the magma generated?

Magma is a silicate melt produced by partial melting

from mantle.

In the depths of the earth,

the mantle is very hot,

but it is applied high pressure.

Thereby the mantle is

in solid state.

If this hot mantle comes up

near the surface of the earth,

then the pressure goes down

and it would be

partially melted.

This is nothing but the magma.

Mantle	High temperature, high pressure, solid
Magma	High temperature but low pressure, liquid

	Where the magma is generated (= where a volcano is formed)
	For the places where the mantle comes up, Refer to the Figure on the previous page.

	(1) Oceanic ridge	Here, the plate tears and the hot mantle comes up from the depths and partially melts which erupts to form a submarine volcano and to become the oceanic crust. Basaltic magma．
	(2) Island arc	An example: the Japanese Archipelago Mantle upwelling induced by subduction of oceanic plate at a trench （Refer to the Figure on the previous page.） Andesite magma, dacite magma (which has high SiO₂ contents and by slow solidification becomes granitic plutons which constitute the continental crust.)
	(3) Hot spot	Examples: the Hawaii Islands, Iceland Mantle gushes up through a tube (pipe-like conduit) from a heat source located much deeper than the plate whose thickness is about 100km.

Mechanism of magma generation

The yellow line

in the figure below shows

the relation between

the temperature

and the depth under the ground.

(Since the pressure

depends on the depth,

we can consider

it as the

relation between

the temperature and the

pressure.)

The temperature of

the mantle is, in general,

lower than

its melting point.

Hence the mantle is solid

usually.

If there is a hot mass of mantle

(temporally denoted

by X in the figure),

the specific gravity of the mass is

slightly lighter

than the surrounding part,

and it gradually goes up

(shown by the pink arrow in the figure).

Because the mantle

is of low heat-conductivity

(not easy to conduct heat),

the mass of mantle hardly

cools while it rises up

(adiabatic rising).

Continuing to rise up,

it would finally begin to melt

due to decompression.

This is nothing

but the magma generation.

Continuing to melt,

the volume of magma increases,

and then the magma is

separated from the mantle.

Because the magma is

light, it continues to go up.

Finally it

reaches the boundary

between the mantle

and the crust, and

piles up there. This is

just the magma chamber.

The magma made thus is

basaltic magma

of which composition is

the same as that of

the submarine crust

or the lava flows on

the Hawaii Islands.

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